Refrigeration units incorporated in refrigerated trailers typically employ both an engine and an electric motor as separate power sources that may be used to drive a compressor in the refrigeration unit. The engine (e.g., a diesel engine) is typically sized having a power output sufficient to meet the temperature pull-down requirements of a particular trailer, while the electric motor is typically sized having a power output sufficient to operate the unit to maintain a particular temperature in the trailer. The power output of the motor is often less than the power output capability of the engine.
Typical electric motors utilized in refrigerated trailer refrigeration units do not have enough power (e.g., 14 hp) to operate the individual components of the unit (e.g., the compressor, an alternator, and fans) at the same speeds, when the unit is operating at a relatively high load, that otherwise are available when the engine is providing power to the system (i.e., when the unit is operating in the high-speed mode with power from the diesel engine). Larger electric motors having higher power outputs and variable-speed capability, which otherwise would be a functional equivalent to the diesel engine in both power output and variable speed operation, are often not used in refrigerated trailer refrigeration units because their size often exceeds the spatial constraints within the refrigeration unit.